Get ready for some big guest stars coming to your favorite shows: Kelly Clarkson will appear on Nashville. A Pitch Perfect star is going to feel right at home on Glee. And there's going to be a Will and Grace reunion on Sean Saves the World. Read on for all the details, including casting news for Hawaii Five-0, Revenge, Arrow, Once Upon a Time and Scandal.

Kelly Clarkson to guest star on Nashville: The inaugural American Idol winner will show up in episode 11 in January. Like with her stint in the Crazy Ones pilot, she'll be playing herself. Showrunner Dee Johnson told TVLine, "As you recall, one of the most popular songs of last season is a song that Gunnar and Scarlett wrote -- 'Fade Into You.' And in fact, in real life, [Clarkson is] actually cutting that song."

A Pitch Perfect Star vs. New Directions on Glee: Skylar Astin, who starred in the hugely popular a capella film, will be a member of the Glee club's competition at Nationals. His character's name is Jean-Baptiste, and he's the leader of the group Throat Explosion. TVLine notes that his episode will air in March.

It's a Will and Grace Reunion on Sean Saves the World: Jack and Karen are back together; okay, not exactly, but the actors who portrayed them. Megan Mullally is set to play Sean's sister in episode 17. But the family members just keep on coming because Entertainment Weekly is reporting that Portia de Rossi will appear in three episodes as his "smart, high-powered ex-wife who re-enters his life." And finally, Guy Pearce has signed on to play a love interest of Sean's in episode 15. TVLine describes his character as a "charming thrill-seeking adventurer who Sean will do almost anything to impress."

Tom Berenger is a former firefighter on Hawaii Five-0: And he just so happens to also be playing Danno's dad. It was previously reported that Melanie Griffith has been cast as Danno's mother. Executive producer Peter Lenkox provided an update on the latter to EW: "[Danny's mom] ends up rocking his world because she comes with this big piece of news, and it forces them to look at their relationship."

Conrad's first wife to cause trouble on Revenge: NYPD Blue alum Gail O'Grady will show up in episode 13 and step into those shoes. Showrunner Sunil Nayar recently said in an interview with E! Online, "That's someone who has always been out there and now we're pretty much planning on having her introduced at some point in time. We're pretty excited about the part and she brings with her secrets that will also blow you guys away that are going to be great."

But that's not all: Brett Cullen is also stopping by Revengeas Jimmy, who is the biological father of Patrick. This will be a recurring role for the actor, and his first episode airs on January 19. According to TVLine, his character is "ruggedly handsome" and "possesses a troubled gaze."

Robert Knepper brings his criminal ways to Arrow: He can currently be seen on Mob City, and others may know him from Heroes or Prison Break, but the actor will continue his mean streak (in character form, that is) when he plays William Tockman in episode 14. Otherwise known as the Clock King, he's a stickler for precision and makes sure that every second is planned out accordingly.

Rebecca Mader goes from Lost to Once Upon a Time: We all remember her as Charlotte on the sci-fi island drama, but now she's playing a villain in multiple episodes. Everything's hush-hush right on now on what fairytale character she's playing, but E! Online notes that she's not only a "major character, [but also has] a major arc, and it's a meaty role." We'll first get to see her in the mid-season finale on December 15.

Once Upon a Time has also cast Rapunzel: Who's playing her in episode 14? Alexandra Metz, who has appeared on The Originals and Chicago Fire. At the moment, as EW reports, she's only scheduled to pop up for that one episode, but she may very well make further appearances down the line.

Jon Tenney is a love interest on Scandal: Things will heat up when The Closer alum shows up as a love interest for First Lady Mellie Grant. This is a recurring role for Tenney, and his stint will occur during the second half of the season when ABC airs the final eight episodes. Not many details are known, but TVLine says that he's playing Andrew, who's in his 40's, and he's being described as handsome and charismatic.